Because they’re evil, mass-marketed offerings from hell for people who secretly hate bookstores and libraries. They provide a cold, disembodied experience which makes marginalia cumbersome if not impossible. In a less slavish age these things would be relegated to SkyMall. Try giving a favorite “book” to someone as a gift through kindle, and you’ll know what I mean.
Because if you really liked reading you would know that a book that is made of paper, ink, and glue is an artifact of history; it’s much more than just words the author wrote. Each book houses an endless world of memory and hastily formed reveries just waiting for you to rediscover each time you read it. Which of your books are falling apart? Which of your books have your name, or the name of an ex-lover, written on the title-page in childish scrawl?